The present invention relates to communications apparatus and methods, more particularly, to apparatus and methods for tracking a communication channel.
Wireless communications systems such as cellular radiotelephone systems typically include a plurality of communication channels which may be established between a first transceiver (such as a base station) and a second transceiver (such as a mobile terminal). The communication channels typically are subject to performance-degrading environmental effects such as multi-path fading and interference (noise). Fading effects include flat fading which may arise from the interaction of a transmitted signal (the main ray) with reflected versions of the transmitted signal that arrive concurrently at a receiver. Time dispersion, another type of fading, may arise from interaction of the main ray with time-delayed reflections of the main ray. Interference effects may be caused by interaction of non-orthogonal signals generated in the signal medium by sources other than the source of the desired transmitted signal. Well-known techniques for reducing the effects of fading include diversity combining of signals from spatially separated antennas. Equalization techniques such as maximum likelihood sequence estimation (MLSE) may be used to compensate for time dispersion. Interference may be reduced by using antenna beam steering to reduce reception of undesired signals.
Fading is typically a major detriment to the performance of demodulators in communication systems. The receiver of a mobile terminal typically includes a demodulator which may be a coherent demodulator such as a maximum likelihood sequence estimator (MLSE) demodulator. To provide for reliable demodulation of a received signal, an associated channel tracker is typically provided for the demodulator. After acquisition of a communicated signal by the receiver, the channel tracker maintains a channel estimate to provide a coherent reference between the demodulator and the received signal.
Unfortunately, however, under certain circumstances, tracking by the channel tracker may be disrupted due to, among other things, fading effects. When tracking is disrupted, the demodulator may lose its coherent reference which may result in errors in the demodulation of the received signal. In a fast fading channel, it is typical to see one or more short deep fades over the duration of a communication burst. A short deep fade tends to cause a burst of errors, even with a high average signal to noise ratio (SNR) as the coherent reference of the demodulator may be lost during the fade. Accordingly, there is a need for improved techniques of channel tracking.
In light of the foregoing, it is an object of the present invention to provide apparatus and methods for channel tracking which are less subject to bursts of errors such as may be caused by fading.
It is another object of the present invention to provide apparatus and methods for channel tracking suitable for use with coherent demodulators providing soft and hard information.
It is yet another object of the present invention to provide apparatus and methods for channel tracking suitable for use with bidirectional demodulation.
These and other objects, features and advantages are provided according to the present invention by apparatus and methods in which variable delay is provided for channel tracking. In particular, a demodulator having both a hard output (or symbol estimate) and a soft output (or indication of the quality of the symbol estimate) is coupled to a channel tracker. The channel tracker provides a channel estimate used by the demodulator, in part, to correct for fading. The channel tracker updates the channel estimate based on the symbol estimates from the demodulator and the received signal. However, when the soft output indicates a low confidence in the symbol estimate, the symbol estimate is not used to update the channel estimate. This is accomplished by providing a variable delay to the channel estimate calculation performed by the channel tracker depending on the number of consecutive suspect symbol estimates. For example, a single suspect estimate generates a delay of one, causing the most recent estimate to not be used, two consecutive suspect estimate generates a delay of two, causing the two most recent estimates to not be used and so on. Accordingly, the channel tracker of the present invention utilizes only the most reliable data available locally to generate a channel estimate for use by the demodulator.
In one aspect of the present invention, a method for tracking a modulated signal transmitted over a communications medium is provided. A modulated signal from a communication channel is received. The received signal is demodulated based on a channel estimate to provide a symbol estimate and an indication of the quality of the symbol estimate. The channel estimate is updated with a variable delay based on the received signal, the symbol estimate and the indication of quality of the symbol estimate. In one embodiment of the present invention the channel estimate is updated by first comparing the indication of quality of the symbol estimate to an acceptance value. An updated channel estimate is generated based on the symbol estimate if the indication of quality of the symbol estimate meets the acceptance value or based on a previous symbol estimate having an indication of quality which meets the acceptance value if the indication of quality of the symbol estimate does not meet the acceptance value.
In a further embodiment of the present invention, the previous symbol estimate has an associated delay relative to the symbol estimate. The updated channel estimate is generated based on a previous symbol estimate having an indication of quality which meets the acceptance by establishing a tracking coefficient based on the associated delay. The updated channel estimate is then generated based on the tracking coefficient. The tracking coefficient for one embodiment is established by raising a base tracking coefficient to the power of the associated delay to provide the tracking coefficient.
In another embodiment of the present invention, the received signal is demodulated by coherently demodulating the received signal using maximum likelihood sequence estimation. The indication of quality of the symbol estimate in this embodiment may be the soft output derived in conjunction with the maximum likelihood sequence estimation. Furthermore, maximum likelihood sequence estimation may include a plurality of candidate states each providing an associated symbol estimate and indication of the quality of the associated symbol estimate. In this case, a separate updated channel estimate is generated for each state based on the associated symbol estimate and indication of the quality of the associated symbol estimate for each state.
The indication of quality of the symbol estimate may also be a received signal strength indication. The channel tracker may be a Kalman-type tracker or a least mean square (LMS) tracker.
While the invention has been described above primarily with respect to the method aspects of the invention, apparatus for performing the methods are also provided. In a further embodiment of the present invention a variable delay channel tracker for a modulated signal receiver apparatus including a demodulator having hard and soft outputs is provided. The channel tracker includes a variable delay channel tracker circuit having a channel estimate output coupled to the demodulator and a delay determination circuit coupled to the soft output of the demodulator and the channel tracker. The variable delay channel tracker circuit may be a Kalman-type tracker or a LMS tracker.
In a further embodiment of the present invention, the delay determination circuit includes means for comparing the soft output to an acceptance value and means for providing an indication to the variable delay channel tracker circuit to update the channel estimate based on the hard output if the soft output meets the acceptance value. The delay determination circuit further includes means for providing a delay count to the variable delay channel tracker circuit for use in generating the channel estimate output if the soft output does not meet the acceptance value.
Accordingly, the methods and apparatus of the present invention provide for improved channel tracking by updating the channel estimate with the best locally available data by the use of a variable delay channel tracker. The channel tracker of the present invention is thereby better able to prevent propogation of errors by channel effects such as deep fading during a transmission.